Chip-shaped electronic components such as multilayer ceramic capacitors or chip resistors are widely used as electronic elements for printed circuit boards or multilayer substrates.
Multilayer ceramic capacitors are typically manufactured by employing following steps: first, conductive paste is applied to green sheets of dielectric ceramic material to form an internal electrodes pattern, the green sheets having internal electrodes pattern are stacked and compressed to form a laminated body and this laminated body is then diced into separate capacitor elements in such a manner that successive internal electrodes are exposed at opposite end portions of the capacitor elements, after which external electrodes are formed on the opposite end portions of the capacitor elements.
When manufacturing chip resistors, first, resistor films such as metal films are disposed on ceramic substrate to form resistor bodies, the surfaces thereof are coated with protecting films and external electrodes are then formed on opposite end portions of the resistor bodies.
These external electrodes are soldered onto solder lands of the printed circuit board in order to connect the electronic components to outer circuit system. And, it is preferred that the external electrodes have heat resistance in order to prevent solder leaching and also solder wettability. These requirements can be fulfilled by using a plurality of layers composed of different metals as external electrodes. Therefore, when forming external electrodes on chip-shaped electronic components such as multilayer ceramic capacitor, chip resistor, or the like, a plurality of electrode layers composed of different materials are disposed on opposite end portions of the electronic components.
As to the multilayer ceramic capacitors, a conductive layer may be formed by applying conductive paste on the opposite end portions of sintered capacitor elements and baking the conductive paste. Or, a conductive layer can be formed by applying conductive paste on the opposite portions of unsintered capacitor elements and co-sintering them. The conductive paste is prepared by mixing an oxidizable metal such as Ni powder with a resin. The surfaces of the metal powder included in the conductive paste are usually oxidized and especially during the baking, they become more seriously oxidized due to the high temperature such that they cannot be electrolytically plated. Therefore, electroless Cu plating or electroless Ni plating has been used in order to form the heat resisting layer and then electrolytic plating is performed on the heat resisting layer.
The electroless Cu plating is performed by immersing an object into a solution containing a reducing agent and a complexing agent. An aqueous solution of sodium hydroxide containing formalin as a reducing agent and ethylenediaminetetraacetic acid (EDTA) as a complexing agent is usually used as the solution for the electroless Cu plating, which causes the production of sodium formate as well as Cu metal during the electroless plating process. Therefore, the treatment of wastewater containing formalin and sodium formate is needed and costs for pollution control systems are incurred.
Further, as nonconductive materials such as plastics can also be plated by electroless plating, electroless plated layers may be formed on the ceramic surfaces as well as on the conductive layers, which is called over-grow, and, when electronic components are soldered onto the printed circuit board having compact wiring patterns, the printed circuit board can be short-circuited by the electroless plated layers on the ceramic surfaces of the electronic elements.